JP4988261B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine, and more particularly to a multi-beam type image forming apparatus that forms an image by exposing a photosensitive member using a plurality of laser beams.

  2. Description of the Related Art Conventionally, as an example of an apparatus that exposes a photosensitive member using a plurality of laser beams and forms an image, there is a multi-beam type image forming apparatus using a rotating polygon mirror (hereinafter referred to as a polygon mirror). In this method, a photosensitive drum is exposed and scanned by generating a plurality of laser beams simultaneously using a polygon mirror. In the case of forming a color image, a plurality of photoconductors arranged at a predetermined interval are exposed with each laser beam, developed, and then transferred onto a sheet sent via a conveyor belt to transfer the color image. I try to record.

  When a color image is formed using a plurality of laser beams in this way, it is necessary to synchronize exposure scanning. If this synchronism is deviated, image distortion and color misregistration occur. For this reason, a BD (Beam Detector) sensor is arranged at the head of laser scanning by a polygon mirror to obtain a synchronizing signal (BD signal) as a scanning start reference, and each color is based on this BD signal. The laser beam writing start timing is determined every time.

  However, in reality, the detection timing of the BD sensor fluctuates due to a change in the rotational balance based on the slight vibration of the rotating body component caused by the positional deviation of the optical elements constituting the optical system, the temperature rise of the polygon mirror part, etc. It was difficult to detect well. For example, when considering the BD signal by the first laser beam as a reference, if the phase of the BD signal by the second laser beam is shifted in the front-rear direction, the timing of writing start of the laser beam of each color is also shifted, which causes the color shift. It will occur. In some cases, the start position of the photoconductor scanning for each color may be shifted by nearly one line.

  Patent Document 1 discloses an example of preventing a start deviation of image exposure in an image forming apparatus using a plurality of laser beams. In this patent document 1, the reference point of each line scan of a plurality of laser beams is detected by a synchronization sensor, a synchronization signal corresponding to each laser beam is generated, and the start timing of image exposure is determined based on each synchronization signal. An image forming apparatus as defined is described. Then, a reference synchronization signal is determined from each synchronization signal, and another synchronization signal whose generation timing is close to that of the reference synchronization signal is delayed by a delay unit to widen the deviation between the plurality of synchronization signals, and image exposure The start timing is determined.

  In this example, it is necessary to define another synchronization signal that is close in generation timing to the reference synchronization signal, and color misregistration may occur in order to widen the shift between the synchronization signals due to delay.

  Further, in Patent Document 2, in an image forming apparatus using a plurality of laser beams, a BD signal by a first laser beam is defined as a reference when generating a horizontal synchronization signal (BD signal), and a laser beam different from that is used. An example is disclosed in which a phase difference with a BD signal is measured in advance and an image writing timing is determined according to the measured phase difference.

In this example, it is necessary to perform an actual measurement of the phase difference in advance, and the phase difference does not always match the predicted value. The positional deviation of the optical elements constituting the optical system, temperature change, and mechanical movement of the polygon mirror The phase difference may change due to slight vibration or the like, resulting in a color shift.
JP 2004-98449 A JP 2004-98299 A

  In a conventional image forming apparatus, when a photosensitive member is exposed using a plurality of laser beams, writing of laser beams of respective colors starts when the phase of the BD signal by the first and second laser beams is shifted in the front-rear direction. However, the timing is shifted and color shift occurs. Further, even in the examples of Patent Documents 1 and 2, there is a possibility that color misregistration occurs, and further improvement is demanded.

 The present invention has been made in view of the above circumstances, and an object thereof is to provide an image forming apparatus that suppresses a deviation in writing start of laser light onto a photosensitive member as much as possible and prevents image deterioration.

  The image forming apparatus according to the first aspect of the present invention is an image forming apparatus that controls a plurality of laser beams based on image data of different colors and exposes and scans a photosensitive member to form an image. The first and second laser light source units that irradiate the mirror with laser beams from different directions and the laser beams from the first and second laser light source units in different optical axis directions by the rotary polygon mirror The first and second optical scanning units that reflect and main-scan the laser beam at a predetermined deflection angle, and the first optical scanning unit are disposed at the incident end in the scanning direction, and the first laser beam is A first beam detector that receives light to detect the start of scanning and an incident end in a scanning direction of the second optical scanning unit, and receives the second laser beam to start scanning. Detect prior to the first beam detector Using the detection results of the second beam detector and the first beam detector, the sampling start timing of the first and second image data is set in common, and the first and second beam detections are performed. A laser control unit for writing the first and second image data in a memory based on the respective detection results of the detector, and controlling the first and second laser light source units based on the image data read from the memory; It is characterized by comprising.

  According to the present invention, when a plurality of laser beams are generated simultaneously using a polygon mirror to perform exposure scanning of a photosensitive drum, so-called line deviation in writing of a laser beam to the photosensitive member is suppressed and image deterioration is suppressed. An image forming apparatus can be provided.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a plan view showing a configuration of an optical system of an image forming apparatus according to an embodiment of the present invention, FIG. 2 is a perspective view schematically showing the optical system of FIG. 1, and FIG. FIG. 2 is a diagram schematically illustrating an internal configuration of an applied tandem color image forming apparatus.

  The image forming apparatus according to the embodiment of the present invention can be applied to a copying machine, a printer, a facsimile machine, and a so-called multi-function peripheral (MFP) that combines these functions.

  In FIG. 1, reference numeral 10 denotes an optical scanning device having a polygon mirror 11 as the center. The polygon mirror 11 is composed of two upper and lower stages, and is shown as rotating in the counterclockwise direction around the rotation axis 11a in the figure. As can be seen from FIG. 2, the upper mirror 111 and the lower mirror 112 are integrated.

  A laser beam is projected onto the polygon mirror 11 via first and second light incident paths arranged substantially symmetrically with respect to the plane including the rotation axis 11 a of the polygon mirror 11. That is, the laser beams from the laser light sources 12K and 12C are projected onto the polygon mirror 11 via the beam splitter 13L. In contrast, the laser beams from the laser light sources 12Y and 12M pass through the beam splitter 13R to the polygon mirror 11. Projected on.

  The laser beam from the laser light source 12C and the laser beam from the laser light source 12K are emitted in parallel from the beam splitter 13L. Similarly, the laser beam from the laser light source 12M and the laser beam from the laser light source 12Y are beams. The light is emitted in parallel from the splitter 13R.

  Laser beams from the respective laser light sources are incident on a plane including the rotation axis 11a of the polygon mirror 11 via a symmetric optical path, reflected by the polygon mirror 11, and laser beams from the laser light sources 12K and 12C. Is corrected by the scanning speed through the first fθ lens 14L and the second fθ lens 15L, and then enters the reflection mirror 16L and is folded. The laser beams from the laser light sources 12Y and 12M pass through the first fθ lens 14R and the second fθ lens 15R and are folded back by the reflection mirror 16R.

  Here, the laser light sources 12Y, 12M, 12C, and 12K project laser beams whose output lights are modulated by image data of Y (yellow), M (magenta), C (cyan), and K (black), respectively. Is. As shown in FIG. 2, of the four laser beams, for example, two laser beams (C, M) are projected onto the upper stage 111 of the polygon mirror 11, and the other two laser beams (K, Y). Is projected to the lower stage 112, the laser beam (K, C) projected through one incident path is set as the first set, and the laser beam (Y, M) projected through the other incident path is set as the second set. As a set, the first set and the second set of beams are reflected by the polygon mirror 11 in a substantially symmetrical direction. As the polygon mirror 11 rotates, the first and second sets of laser beams are scanned in the directions indicated by dotted lines A and B in FIG.

  As shown in FIG. 2, the first fθ lenses 14L and 14R, the second fθ lenses 15L and 15R, and the reflection mirrors 16L and 16R also have a two-stage configuration of an upper stage and a lower stage, and laser light sources C and M The laser beam output from the laser beam passes through the upper path, and the laser beams output from the laser light sources K and Y pass through the lower path.

  Further, the laser beams folded back by the reflection mirrors 16L and 16R expose the photosensitive drums for K, C, M, and Y, respectively, via an optical system (reflection mirror or the like) (not shown) (described later). .

  Further, two laser beams (K, C and Y, M) of each set are detected in the overscan area (area that does not contribute to exposure) in the vicinity of the scanning start ends of the reflection mirrors 16L and 16R. Beam detectors 17L and 17R are provided. The beam detector 17L includes a reflection mirror 18L and an optical sensor 19L. Similarly, the beam detector 17R includes a reflection mirror 18R and an optical sensor 19R.

  A synchronization signal for synchronizing the laser beam (laser beam) in the main scanning direction can be generated using the detection signals (BD signal) from the beam detectors 17L and 17R. Here, each of the optical sensor 19L and the optical sensor 19R includes two optical detection devices that are optically and electrically isolated in order to independently detect the laser beams K and C and the laser beams Y and M, respectively. .

  As shown in FIG. 1, the optical scanning device 10 is basically configured symmetrically with respect to the plane including the rotation axis 11a of the polygon mirror 11, and further the interval between the beam detector 17L and the reflection mirror 16L is set. L1, L1 <L2 is established when the distance between the beam detector 17R and the reflecting mirror 16R is L2.

  Here, when L1 = L2, if the manufacturing error is not included, when the polygon mirror 11 rotates, the laser beam enters the reflection mirror 18R simultaneously with the reflection mirror 18L and is detected by the beam detector 17L. It is assumed that detection by the beam detector 17R is performed simultaneously.

  By setting L1 <L2 as described above, when the polygon mirror 11 rotates, the laser beam is incident on the reflection mirror 18R before the reflection mirror 18L, and the detector is more effective than the detection by the beam detector 17L. The detection at 17R always precedes. That is, when the scanning optical system is arranged symmetrically, the optical sensors 19L and 19R are slightly shifted from the symmetrical positions, and the optical sensor 19R receives the laser beam in advance before the optical sensor 19L receives the laser beam. Like to do.

  The difference between L2 and L1 is small due to a shift in the fixed position of the optical element constituting the scanning optical system at the time of manufacture, a change in the position of the optical element based on a change in environmental conditions such as temperature, and a temperature rise in the polygon mirror part In consideration of changes in the rotational balance due to vibration, etc., it is determined that detection by the detector 17R always precedes detection by the beam detector 17L.

  Note that the positions where the beam detectors 17L and 17R are provided are not limited to the illustrated example, and may be arranged near the scanning start ends of the fθ lenses 15L and 15R, for example. However, also in this case, the detectors are arranged so that detection by the beam detector 17R precedes. The reason why the detection by the beam detector 17R is preceded will be described in the circuit operation described later.

  In the illustrated example, the two laser beams are output in parallel using the beam splitters 13L and 13R, but an optical system such as a reflection mirror may be used instead of the beam splitter.

  FIG. 3 schematically shows an internal configuration of a tandem color image forming apparatus 20 using a laser beam folded back by the reflection mirrors 16L and 16R. Since the image forming apparatus 20 forms an image of each color of K (black), C (cyan), M (magenta), and Y (yellow), four image forming units 21K, 21C, 21M, and 21Y are transferred. Arranged along the paper movement direction X. These image forming units 21K, 21C, 21M, and 21Y have photosensitive drums 22K, 22C, 22M, and 22Y, respectively, and the rotational axes of the photosensitive drums 22K, 22C, 22M, and 22Y are parallel to the main scanning direction. And arranged in a line at a predetermined pitch interval in the transfer paper movement direction X (sub-scanning direction).

  The image forming units 21K, 21C, 21M, and 21Y have the same configuration. The image forming unit 21K will be described as a representative example. The charger 23K, the developing unit 24K, and the transfer unit 25K arranged around the photosensitive drum 22K. Etc. Similarly, the image forming units 21M, 21C, and 21Y have a charger, a developing device, a transfer device, and the like. The conveyor belt 26 is installed between the driving roller 27 and the driven roller 28 and is driven to rotate in the direction of the arrow X (sub-scanning direction) shown in the figure. Paper cassettes 29 and 30 are provided below the conveyor belt 26. It has been.

  The transfer paper is first conveyed to the image forming unit 21Y, where yellow image formation is performed. The surface of the photosensitive drum 22Y is uniformly charged by a charger and then exposed to a laser beam output from a laser light source 12Y modulated by yellow image data to form an electrostatic latent image. The electrostatic latent image formed on the photosensitive drum 22Y is developed by a developing device, and a yellow toner image is formed on the photosensitive drum 22Y. The toner image is transferred by a transfer device at a position (transfer position) where the photosensitive drum 22Y and the transfer belt 26 are in contact with the transfer paper, thereby forming a yellow image on the transfer paper. After the transfer, the photosensitive drum 22Y cleans unnecessary toner remaining on the drum surface and prepares for the next image formation.

  Thus, the transfer sheet on which the yellow image is transferred by the image forming unit 21Y is transported to the next image forming unit 21M by the transport belt 26. Similarly, the magenta, cyan, and black toner images are transferred in a superimposed manner, and the transfer paper on which the color image is formed is peeled off from the transport belt 26 and discharged.

  Further, the image forming apparatus 20 is provided with a laser exposure unit 31 by laser scanning. The laser exposure unit 31 includes the optical scanning device 10 described with reference to FIGS. 1 and 2, and laser beams for Y, M, C, and K are respectively applied to the surfaces of the photosensitive drums 22Y, 22M, 22C, and 22K in the main scanning direction. Irradiate while scanning.

  Further, the image forming apparatus 20 is provided with a color scanner unit 32. A transparent manuscript table 33 is provided above the color scanner unit 32. The color scanner unit 32 reads a color image of the manuscript placed on the manuscript table 33 using an image sensor or the like, and R (red). , G (green), and B (blue) are converted into electrical signals corresponding to the three primary colors.

  The R, G, and B signals are converted into Y (yellow), M (magenta), C (cyan), and K (black) color signals to generate image data of C, M, Y, and K colors. The The laser light source of the laser exposure unit 31 is controlled based on the image data of each color.

  Further, in order to detect the paper feeding state of the paper conveyed from the paper feeding cassettes 29 and 30, it is near the driven roller 28. A photo sensor 34 is provided to detect the start of paper feeding. The photo sensor 34 is not limited to the illustrated position, and may be any position that can detect the start of paper feeding.

  The laser light source of the laser exposure unit 31 can be controlled not only by the image data obtained by the color scanner unit 32 but also by image data created by a personal computer (PC) or the like.

  Next, the configuration of the image processing unit in the image forming apparatus according to the embodiment of the present invention will be described.

  FIG. 4 is a block diagram illustrating a configuration of the image processing unit 40. The image processing unit 40 includes a reading device 41 that reads image data, an image data processing unit 42, page memory controllers 43L and 43R, and laser control devices 44L and 44R.

  Here, the page memory controller 43L and the laser control device 44L are, for example, devices for cyan (C) or black (K), and the page memory controller 43R and the laser control device 44R are, for example, magenta (M) or yellow. (Y) is an apparatus. For the sake of brevity, in the following description, the page memory controller 43L and the laser control device 44L will be described as C or K devices, and the page memory controller 43R and the laser control device 44R will be described as M or Y devices. To do.

  Image data from the reading device 41 is supplied to a bus line 401 via an interface (I / F) 46, and a controller (CPU) 47 and an image memory 48 are connected to the bus line 401, and further created by a PC 49 or the like. The image data is supplied to the bus line 401 via the LAN and I / F 50.

  The laser controllers 44L and 44R control the laser light sources 61L and 61R, and detection signals (BD signals) are input from the optical sensors 19L and 19R of the beam detectors 17L and 17R.

  The reading device 41 includes, for example, the color scanner unit 32, photoelectrically converts an image of a document with a CCD or the like, outputs RGB image data, and compresses and stores it in an image memory 48. When the image data stored in the image memory 48 is printed out, the image data processing unit 42 performs an expansion process, converts the RGB image data into a YMCK signal, performs color space conversion, and performs gamma correction, gradation correction, and the like. Image quality correction is performed, and the image data of each color is transferred to the page memory controllers 43L and 43R.

  In the block diagram shown in FIG. 4, for example, C (cyan) image data is output to the page memory controller 43L, M (magenta) image data is output to the page memory controller 43R, or K (black) image. Data is output to the page memory controller 43L and Y (yellow) image data is output to the page memory controller 43R. The reason why the compression process is performed when the image data is stored in the image memory 48 is to effectively use the memory, and the decompression process is performed after reading.

  The image data transferred to the page memory controllers 43L and 43R are pulse width modulated by the laser controllers 44L and 44R, respectively, and output to the laser light sources 61L and 61R, respectively, to control the laser. The laser light source 61L corresponds to, for example, the laser light source 12C or 12K in FIGS. 1 and 2, and the laser light source 61R corresponds to the laser light source 12M or 12Y.

  Laser beams from the respective laser light sources irradiate the photosensitive drums 22 of the image forming units 21C and 21M or 21K and 21Y of FIG. 3 through the optical systems of FIGS. The page memory controllers 43L and 43R read image data from the image data processing unit 42 to the laser control devices 44L and 44R from the image data processing unit 42 based on the detection signal BD (L) from the optical sensor 19L. That is, the page memory controllers 43L and 43R generate an enable signal based on the synchronization signal (H sync) generated based on the detection signal BD (L) and the output of the photo sensor 34 that detects the start of paper feeding, The image data is transferred to the laser control devices 44L and 44R.

  FIG. 5 shows a specific configuration example of the laser control devices 44L and 44R. The laser control device 44L includes a line buffer memory 51L, a memory controller 52L that controls data transfer to the line buffer memory 51L, and a pulse width shaping circuit 53L to which the detection signal BD (L) from the optical sensor 19L is input. A PWM (Pulse Width Modulation) circuit 54L that performs pulse width modulation on the data read from the line buffer memory 51L is provided. In the description of FIG. 5, the C (cyan) and K (black) laser beams are detected by the sensor 19L, and the M (magenta) and Y (yellow) laser beams are detected by the sensor 19R.

  The BD (L) signal detected by the optical sensor 19L is input to the pulse width shaping circuit 53L, shaped into a waveform with a predetermined pulse width, and converted into a synchronization signal H sync (L) that matches the timing of the laser controller 44L. Is done. Here, in order to synchronize the timing of the synchronizing signal H sync (L) with the laser controller 44L, a pulse clock (P clock) used in the main part of the laser controller 44L is input to the pulse width shaping circuit 53L.

Next, the synchronization signal H sync (L) and the main clock signal (m Clock) are input to the flip-flop 57L. Here, the main clock signal (m Clock) is a clock used in the page memory controllers 43L and 43R, and the synchronization signal H sync (L) whose timing is reset by the main clock signal (m Clock) is the page memory controller 43L. And input to the page memory controller 43R. It should be noted that the synchronization signal H sync (L) is input not only to the page memory controller 43L but also to the page memory controller 43R. When the synchronization signal H sync (L) is sent to the page memory controllers 43L and 43R. Then, image data can be transmitted from the image data processing unit 42, an enable signal (DATA Enable) is generated in response to the input of the synchronization signal H sync (L), and an image is read into the memory controller 52L in units of pages. It is. As described above, the page memory controllers 43L and 43R are operated by the common main clock signal (m Clock) and are synchronized with the synchronization signal H sync (L) generated by the flip-flop 57L.

  Further, as described above, the main part of the laser control device 44L operates with a pulse clock (P Clock), and a pulse clock signal (P Clock) is supplied to the memory controller 52L and the PWM circuit 54L, respectively. Further, for example, C (cyan) image data (DATA C) is transferred from the page memory controller 43L to the laser controller 44L.

  The enable signal (DATA Enable) input from the page memory controller 43L to the laser control device 44L becomes, for example, a high level when image data transfer is impossible, and becomes a low level when image data transfer is possible. The memory controller 52L receives a Low enable signal indicating that image data can be transferred, and reads and reads parallel data for one page of C image data for each line. The memory controller 52L converts the read image data into serial data, and writes data for one line into the line buffer memory 51L.

  Further, the memory controller 52L sequentially reads out the image data in the line buffer memory 51L at a predetermined timing based on H sync (L) and transfers it to the PWM circuit 54L.

  Writing to the line buffer memory 51L is performed according to the L sampling signal synchronized with the synchronization signal H sync (L).

  Note that the transfer period of the color image data for one page is determined based on a paper feed start signal obtained from the photosensor 34 (FIG. 3). The transfer period of one page of color image data corresponds to the C image exposure period.

  The PWM circuit 54L performs pulse width modulation according to the level of the image data, and supplies a laser control signal to the laser light source 61L for C (cyan). The PWM circuit 54L operates by a pulse clock (Pclock), and outputs a signal whose pulse width changes in accordance with the value of C of the pixel within the period of the pulse clock.

  On the other hand, the laser controller 44R has the same configuration, and includes a line buffer memory 51R, a memory controller 52R, a pulse width shaping circuit 53R to which a detection signal BD (R) from the optical sensor 19R is input, and a PWM circuit 54R. It has.

  The pulse width shaping circuit 53R receives the detection signal BD (R) and the pulse clock signal (P clock) from the optical sensor 19R, and the generation timing of the first clock signal when the BD (R) signal is input. A synchronization signal H sync (R) shaped into a waveform with a predetermined pulse width is generated in accordance with The above points are the same as those of the laser control device 44L. However, the laser control device 44R is different from the laser control device 44L in the following points.

  As described with reference to FIGS. 1 and 2, the detection signal BD (R) from the optical sensor 19R is set to be generated prior to the detection signal BD (L) from the optical sensor 19L. Therefore, the phase of the synchronization signal H sync (R) created based on the detection signal BD (R) is advanced than that of the synchronization signal H sync (L).

  Also, the page memory controller 43R receives the synchronization signal H sync (L) generated by the laser control device 44L as a reference synchronization signal, so that the image data processing device 42 receives, for example, M (magenta). Image data can be transmitted, and an enable signal (DATA Enable) is supplied from the page memory controller 43R to the memory controller 52R of the laser controller 44R. The memory controller 52R is supplied with a pulse clock signal (P Clock) and, for example, M image data (DATA M) is read.

  The memory controller 52R of the laser control device 44R receives the enable signal of the image creation instruction level (Low) from the page memory controller 43R, samples and reads parallel data for one page of the M image data, and converts it into serial data. Then, the data for one line is written into the line buffer memory 51R. In parallel with this, the memory controller 52R sequentially reads out the image data in the line buffer memory 51R at a predetermined timing based on H sync (R) and transfers it to the PWM circuit 54R. Writing to the line buffer memory 51R is performed according to the R sampling signal synchronized with the synchronization signal H sync (R).

  The writing period of one page of M image data to the laser control device 44R is the same as the writing period of one page of C image data to the laser control device 44L, but the timing is different. The transfer period of the color image data for one page corresponds to the exposure period of the M image.

  The K (black) image data is processed using the same circuit as the page memory controller 43L and the laser controller 44L, and the Y (yellow) image data is processed by the page memory controller 43R and the laser controller 44R. A similar circuit is used.

  Next, the operation of the image forming apparatus according to the embodiment shown in FIG. 5 will be described with reference to FIG. The logic signal in FIG. 6 is low and active.

The waveform shown in FIG. 6A shows the synchronization signal H sync (L) generated based on the detection signal BD (L) from the optical sensor 19L, and the waveform shown in FIG. 6B shows the page memory controller. An enable signal (DATA Enable) output from 43L and input to the memory controller 52L is shown. FIG. 6C shows a sampling period L sampling in which the memory controller 52 in the laser control device 44L reads image data.
On the other hand, FIG. 6D shows the synchronization signal H sync (R) obtained based on the detection signal BD (R) from the sensor 19R in the laser control device 44R, and FIG. 6E shows the laser control device. A sampling period R sampling in which the memory controller 52R in 44R reads image data is shown.

  As shown in FIG. 6B, the enable signal (DATA Enable) from the memory controller 43L is generated in response to the generation of the synchronization signal H sync (L) in (a), and is sent from the page memory controllers 43L and 43R. Image data (DATA L) and (DATA R) are also output to the laser control devices 44L and 44R at the same timing as the enable signal (DATA Enable).

  The period during which the enable signal (DATA Enable) is at the high level is a period for masking the transfer of the image data from the page memory controller. When the enable signal is at the low level, the image data can be transferred. When the enable signal (DATA Enable) becomes Low level and the first synchronization signal H sync (L) is received, the sampling of the image data DATA (L) is started at the timing t1 in FIG. 6C, and the line buffer memory 51L. Data is written to the.

  On the other hand, since the sensor 19R in the laser control device 44R always detects the laser beam before the sensor 19L in the laser control device 44L, as shown in FIG. 6D, the synchronization signal H sync (R) is Occurs before the sync signal H sync (L).

  Therefore, as shown in FIG. 4E, the sampling period R sampling of the image data in the memory controller 52R precedes the L sampling in (c) by the time Δt, and the image data DATA (R) from the page memory controller 43R Sampling is started and data is written into the line buffer memory 51R.

  FIG. 7 is a timing chart for explaining operations in a conventional general image forming apparatus, and shows an example of a type in which the beam detectors 17L and 17R detect a laser beam at the same timing. In this case, there is no problem if BD (L) and BD (R) always occur at the same timing, but which of BD (L) and BD (R) occurs first due to an assembly error of the optical system or the like. In addition, the components constituting the rotating body may slightly vibrate due to the temperature rise of the polygon mirror portion and the rotation balance changes, and the detection timing by the BD sensor may fluctuate.

  That is, when the sampling period is set based on one sync signal, for example, H sync (L), the sync signal H sync (R) is synchronized with the sync signal H sync (L) as shown in FIG. When the phase is shifted forward, R sampling may be slightly shifted from L sampling as shown by the solid line in (e). However, when the phase of the sync signal H sync (R) is shifted backward with respect to the sync signal H sync (L), as shown by the dotted line in (e), R sampling is almost one line than L sampling. It will shift forward. As a result, since the scanning positions of the first laser beam and the second laser beam are shifted, the color blur becomes conspicuous when printed out.

  In the present invention, even when the sampling period is set with reference to one of the synchronization signals, for example, H sync (L), the detection by the sensor 19R always precedes the detection by the sensor 19L. Is solved. Therefore, it is possible to prevent image deterioration due to a shift in writing start.

  Next, another embodiment of the image forming apparatus of the present invention will be described with reference to FIG. In the previous embodiment, the example in which the synchronization signal H sync (R) is generated prior to H sync (L) by the position of the BD sensors 17L and 17R, that is, the mechanical structure arrangement has been described. In the embodiment, the synchronization signal H sync (R) is generated prior to H sync (L) by electrical processing.

  FIG. 8 shows a main part of the laser control devices 44L and 44R. The laser control device 44L inputs the detection signal BD (L) from the sensor 19L to the Schmitt buffer 56L via the time constant circuit 55L including the resistor R1 and the capacitor C1, and the detection signal BD (L) is output by the time constant circuit 55L. And is output via the Schmitt buffer 56L, and the delayed detection signal BD (L) is input to the pulse width shaping circuit 53L.

  The other laser control device 44R inputs the detection signal BD (R) from the BD sensor 19R to the pulse width shaping circuit 53R via the Schmitt buffer 56R. Other configurations are the same as those in FIG.

  In this example, the synchronization signal H sync (L) generated based on the delayed detection signal BD (L) is supplied to the page memory controllers 43L and 43R as a reference signal. Therefore, since the other synchronization signal H sync (R) always advances in phase with respect to the synchronization signal H sync (L), image data is sampled at the timings shown in FIGS. The period R sampling precedes L sampling by a time Δt, and data is written to the line buffer memory 51R. By providing an appropriate time constant circuit 55L, it is possible to prevent a shift of nearly one line as shown in FIG.

  In addition, although the example using the time constant circuit 55L and the Schmitt buffer 56L has been described as the delay means, other delay circuits may be used. In short, it is sufficient that the delay time can be set so that the synchronization signal H sync (R) always precedes the reference synchronization signal H sync (L). However, if the delay time is too long, a shift of nearly one line occurs as shown in FIG. 7E, so the phase difference Δt needs to be an appropriate value. For example, a counter circuit that uses a clock signal (P Clock) to delay by several clocks may be employed. As the clock signal (P Clock), for example, about 50 MHz is used.

  As described above, in the present invention, when a plurality of laser beams are simultaneously generated using a polygon mirror to expose and scan a photosensitive drum, so-called line deviation in writing of the laser beam to the photosensitive member is suppressed and image degradation is caused. An image forming apparatus can be provided.

  In the above description, the synchronization signal obtained based on the detection signal BD (L) signal from the BD sensor 19L by advancing the synchronization signal H sync (R) over the synchronization signal H sync (L). The example of supplying to the page memory controllers 43L and 43R has been described. However, conversely, a synchronization signal obtained based on the detection signal BD (R) signal from the BD sensor 19R may be supplied to the page memory controllers 43L and 43R.

  In addition, it is not an essential problem whether to advance the phase of the synchronization signal H sync (R) or the synchronization signal H sync (L), and the synchronization signal H sync (L) is higher than the synchronization signal H sync (R). May be advanced so that the phase relationship between the synchronization signal H sync (R) and the synchronization signal H sync (L) does not fluctuate.

 Various modifications can be made without departing from the scope of the claims.

1 is a plan view showing a configuration of an optical scanning device used in an image forming apparatus according to an embodiment of the present invention. 1 is a perspective view illustrating a configuration of an optical scanning device used in an image forming apparatus according to an embodiment of the present invention. 1 is an explanatory diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a block diagram illustrating an image processing unit of an image forming apparatus according to an embodiment of the present invention. FIG. 5 is a block diagram mainly illustrating a laser control device in the image processing unit of FIG. 4. 6 is a timing chart for explaining the operation of a synchronous system in the image forming apparatus of the present invention. 6 is a timing chart for explaining the operation of a synchronous system in a general image forming apparatus. FIG. 6 is a block diagram for explaining an image processing unit of an image forming apparatus according to another embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Optical scanning device 11 ... Polygon mirror 12 ... Laser light source 13 ... Beam splitter 14L, 14R, 15L, 15R ... f (theta) lens 16L, 16R ... Reflection mirror 17L, 17R ... Beam detector 18L, 18R ... Reflection mirror 19L, 19R ... Optical sensor 20 ... Image forming device 40 ... Image processing unit 41 ... Reading device 42 ... Image data processing unit 43L, 43R ... Page memory controller 44L, 44R ... Laser control device 51L, 51R ... Line buffer 52L, 52R ... Memory controller 53L, 53R ... Pulse width shaping circuits 54L, 54R ... PWM circuit 55L ... Time constant circuits 56L, 56R ... Schmitt buffer 57L ... Flip-flop

Claims (5)

An image forming apparatus that controls a plurality of laser beams based on image data of different colors and exposes and scans a photoreceptor to form an image,
First and second laser light source units for irradiating the rotary polygon mirror with laser beams from different directions;
A first and a second optical scanning section for reflecting the laser beams from the first and second laser light source sections in different optical axis directions by the rotary polygon mirror and performing main scanning at a predetermined deflection angle; ,
A first beam detector disposed at an incident end in a scanning direction of the first optical scanning unit and receiving the first laser beam to detect the start of scanning;
A second beam disposed at an incident end in the scanning direction of the second optical scanning unit and receiving the second laser beam to detect the start of scanning ahead of the first beam detector; A detector;
Using the detection result of the first beam detector, the sampling start timings of the first and second image data are set in common, and based on the detection results of the first and second beam detectors. And a laser control unit for writing the first and second image data into the memory and controlling the first and second laser light source units based on the image data read from the memory. Forming equipment.   The first and second beam detectors are respectively disposed in an overscan region, and the second beam detector is disposed before the first beam detector receives the first laser beam. 2. The image forming apparatus according to claim 1, wherein positions of the first and second beam detectors are set so as to receive the second laser beam.   The rotary polygon mirror has mirror surfaces arranged in multiple stages above and below, and the first and second laser light source units are arranged in the vertical direction, and a plurality of laser beams of different colors are incident on the multi-stage mirror surfaces in parallel. The image forming apparatus according to claim 1, further comprising a laser light source. Furthermore, based on the detection results by the first and second beam detectors, including a synchronization signal generation circuit for generating first and second synchronization signals corresponding to each laser beam,
The sampling start timing of the first and second image data is set in common based on the first synchronization signal, and the first and second image data are set based on the first and second synchronization signals. The image forming apparatus according to claim 1, wherein the image forming apparatus writes in a memory.   The image forming apparatus according to claim 1, wherein the image forming apparatus includes a plurality of photosensitive members exposed by laser beams from a plurality of laser light sources, and is configured in a tandem system.

Images